Peru -- Primarily Machu Picchu (Dec 2010)

Guide is pointing to a horseshoe-shaped groove cut into two adjacent blocks. In the original sturcture, they poured molten gold into these grooves. The gold would solidify forming sort of a hook-shaped metal bar that acted to tie the two stones together. This helped the wall withstand sideways shaking forces during earthquakes. They think some of these gold "tie rods" are still present in some wall secions that were never torn down (but are now in the foundations of large structures).